Saving procedural MeshInstance3D backed by ArrayMesh as resource or scene causes access violation

Tested versions

Reproduced

4.2.2-stable, 4.1.4-stable

Not repro'd

4.3-dev5

System information

Godot v4.2.2.stable.mono - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4080 (NVIDIA; 31.0.15.5222) - 11th Gen Intel(R) Core(TM) i7-11700K @ 3.60GHz (16 Threads)

Issue description

Hey team! Working in a Mono-enabled version of Godot here for performance. Attached MRP uses C#, I'm steadily working to convert the core to GDScript for easier access; but the access violation below originates from within the native ResourceSaver::save call somewhere from what I could trace, so it should be agnostic.

This appears in latest 4.2.2-stable, but surprisingly doesn't appear in 4.3-dev5 (i.e. what I'm attempting just works there). Curious to know where the delta is!

Context

For a side project, I generate lots of procedural mesh chunks (with LOD info) in a grid. My latest move was to serialize the generated mesh data, so that I don't have to regenerate from map data every time.

I devised two ways to do this:

Directly save the ArrayMesh resources pulled from MeshInstance3Ds to a PackedMap custom resource (list or dict), so it can be reproduced by a packed map loader in a future dynamic scene.
Create a new Node3D, duplicate + add_child + set owner on all of the generated MeshInstance3Ds from the map generator node, and then pack it into a scene with metadata (PackedMap).

The Bug

Having tried both ways (and settled on the latter), I ran into a brick wall:

Method 1, upon ResourceSaver.Save(packedMap, path), would spin for a little bit, and then produce the following native error:


Fatal error. System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
at Godot.NativeInterop.NativeFuncs.godotsharp_method_bind_ptrcall(IntPtr, IntPtr, Void**, Void*)
at Godot.NativeCalls.godot_icall_3_984(IntPtr, IntPtr, IntPtr, System.String, Int32)
at Godot.ResourceSaver.Save(Godot.Resource, System.String, SaverFlags)
at GGS.Map.SaveMap()
at GGS.Map.InvokeGodotClassMethod(Godot.NativeInterop.godot_string_name ByRef, Godot.NativeInterop.NativeVariantPtrArgs, Godot.NativeInterop.godot_variant ByRef)
at Godot.Bridge.CSharpInstanceBridge.Call(IntPtr, Godot.NativeInterop.godot_string_name*, Godot.NativeInterop.godot_variant**, Int32, Godot.NativeInterop.godot_variant_call_error*, Godot.NativeInterop.godot_variant*)

Godot Editor stopped with exit code: 3221225477 (-> hex 0xC0000005)


- Method 2 (shown in the MRP) would also spin for a bit, with a corresponding memory increase, and produce the above error.

I have not yet worked with a scratch-built local copy of Godot (but particularly 4.2.2-stable) so I could natively debug it and diagnose further, but figured I would push this here.

Even if this is a no-factor for 4.3 (as it appears resolved in dev5), I want to determine where the source fix was to learn from it!

### Steps to reproduce

1. Open MRP with a console-enabled version of Godot (e.g. console exe, via VSCode)
2. `test_mapgen` scene should take a second to open by default -- if not, open it
3. Click on the `Map` node, and trigger the `Save Map To Disk` flag
4. Wait for the crash...
5. Observe the above access violation

### Minimal reproduction project (MRP)

[MeshCrashMiniRepo.zip](https://github.com/godotengine/godot/files/15172411/MeshCrashMiniRepo.zip)

I'm able to reproduce in 4.2.2.stable.mono.custom_build.15073afe3, but as mentioned it seems to be fixed in master. Here's the stacktrace anyway:

* thread #1, name = 'godot.linuxbsd.', stop reason = signal SIGSEGV: address not mapped to object (fault address: 0x7fbd8f801000)
  * frame #0: 0x000055555faf7fa8 godot.linuxbsd.editor.dev.x86_64.llvm.mono`FileAccessCompressed::store_8(this=0x000055557c981070, p_dest='\0') at file_access_compressed.cpp:349:25
    frame #1: 0x000055555faee45b godot.linuxbsd.editor.dev.x86_64.llvm.mono`FileAccess::store_buffer(this=0x000055557c981070, p_src="\xe8\U00000003", p_length=1488024) at file_access.cpp:711:3
    frame #2: 0x000055555fb6cb72 godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceFormatSaverBinaryInstance::write_variant(f=Ref<FileAccess> @ 0x00007fffffffa3a8, p_property=0x00005555e0805f68, resource_map=0x00007fffffffbdf0, external_resources=0x00007fffffffc2e0, string_map=0x00007fffffffc2a0, p_hint=0x00007fffffffa378) at resource_format_binary.cpp:1841:7
    frame #3: 0x000055555fb6c9e5 godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceFormatSaverBinaryInstance::write_variant(f=Ref<FileAccess> @ 0x00007fffffffacf0, p_property=0x00005555e0806258, resource_map=0x00007fffffffbdf0, external_resources=0x00007fffffffc2e0, string_map=0x00007fffffffc2a0, p_hint=0x00007fffffffacc0) at resource_format_binary.cpp:1831:5
    frame #4: 0x000055555fb6c78d godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceFormatSaverBinaryInstance::write_variant(f=Ref<FileAccess> @ 0x00007fffffffb5a8, p_property=0x00005555e08062f0, resource_map=0x00007fffffffbdf0, external_resources=0x00007fffffffc2e0, string_map=0x00007fffffffc2a0, p_hint=0x00007fffffffb578) at resource_format_binary.cpp:1822:5
    frame #5: 0x000055555fb6c9e5 godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceFormatSaverBinaryInstance::write_variant(f=Ref<FileAccess> @ 0x00007fffffffbcd0, p_property=0x00005555e0806338, resource_map=0x00007fffffffbdf0, external_resources=0x00007fffffffc2e0, string_map=0x00007fffffffc2a0, p_hint=0x00005555e0806350) at resource_format_binary.cpp:1831:5
    frame #6: 0x000055555fb71800 godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceFormatSaverBinaryInstance::save(this=0x00007fffffffc240, p_path=0x00007fffffffc318, p_resource=0x00007fffffffc558, p_flags=32) at resource_format_binary.cpp:2288:4
    frame #7: 0x000055555fb733e7 godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceFormatSaverBinary::save(this=0x0000555560a8f050, p_resource=0x00007fffffffc558, p_path=0x00007fffffffc448, p_flags=32) at resource_format_binary.cpp:2427:15
    frame #8: 0x000055555fb81719 godot.linuxbsd.editor.dev.x86_64.llvm.mono`ResourceSaver::save(p_resource=0x00007fffffffc558, p_path=0x00007fffffffc828, p_flags=32) at resource_saver.cpp:128:19
    frame #9: 0x000055555f8e69e7 godot.linuxbsd.editor.dev.x86_64.llvm.mono`core_bind::ResourceSaver::save(this=0x0000555560cdeb40, p_resource=0x00007fffffffc558, p_path=0x00007fffffffc828, p_flags=(value = 32)) at core_bind.cpp:154:9
    frame #10: 0x000055555f977701 godot.linuxbsd.editor.dev.x86_64.llvm.mono`void call_with_ptr_args_ret_helper<__UnexistingClass, Error, Ref<Resource> const&, String const&, BitField<core_bind::ResourceSaver::SaverFlags>, 0ul, 1ul, 2ul>(p_instance=0x0000555560cdeb40, p_method=(godot.linuxbsd.editor.dev.x86_64.llvm.mono`core_bind::ResourceSaver::save(Ref<Resource> const&, String const&, BitField<core_bind::ResourceSaver::SaverFlags>) at core_bind.cpp:152), p_args=0x00007fffffffc7b0, r_ret=0x00007fffffffc830, (null)=IndexSequence<0UL, 1UL, 2UL> @ 0x00007fffffffc588) at binder_common.h:334:22
    frame #11: 0x000055555f977615 godot.linuxbsd.editor.dev.x86_64.llvm.mono`void call_with_ptr_args_ret<__UnexistingClass, Error, Ref<Resource> const&, String const&, BitField<core_bind::ResourceSaver::SaverFlags>>(p_instance=0x0000555560cdeb40, p_method=(godot.linuxbsd.editor.dev.x86_64.llvm.mono`core_bind::ResourceSaver::save(Ref<Resource> const&, String const&, BitField<core_bind::ResourceSaver::SaverFlags>) at core_bind.cpp:152), p_args=0x00007fffffffc7b0, r_ret=0x00007fffffffc830) at binder_common.h:582:2
    frame #12: 0x000055555f976b95 godot.linuxbsd.editor.dev.x86_64.llvm.mono`MethodBindTR<Error, Ref<Resource> const&, String const&, BitField<core_bind::ResourceSaver::SaverFlags>>::ptrcall(this=0x0000555560cdef70, p_object=0x0000555560cdeb40, p_args=0x00007fffffffc7b0, r_ret=0x00007fffffffc830) const at method_bind.h:513:3
    frame #13: 0x000055555b342bae godot.linuxbsd.editor.dev.x86_64.llvm.mono`godotsharp_method_bind_ptrcall(p_method_bind=0x0000555560cdef70, p_instance=0x0000555560cdeb40, p_args=0x00007fffffffc7b0, p_ret=0x00007fffffffc830) at runtime_interop.cpp:557:17
    frame #14: 0x00007fff4965d8db
    frame #15: 0x00007fff4965d7e1
    frame #16: 0x00007fff4bf3568b
    frame #17: 0x00007fff4bf350fe
    frame #18: 0x00007fff4c3eb357
    frame #19: 0x00007fff4c3e8ae3
    frame #20: 0x00007fff4964b9bc
    frame #21: 0x000055555b322956 godot.linuxbsd.editor.dev.x86_64.llvm.mono`CSharpInstance::callp(this=0x000055557497ba50, p_method=0x00007fffcc04d320, p_args=0x0000000000000000, p_argcount=0, r_error=0x00007fffffffd198) at csharp_script.cpp:1768:2
    frame #22: 0x00005555600f5157 godot.linuxbsd.editor.dev.x86_64.llvm.mono`Object::callp(this=0x0000555579cb05e0, p_method=0x00007fffcc04d320, p_args=0x0000000000000000, p_argcount=0, r_error=0x00007fffffffd198) at object.cpp:753:26
    frame #23: 0x000055555fcfc3a4 godot.linuxbsd.editor.dev.x86_64.llvm.mono`Callable::callp(this=0x00007fffcc04d320, p_arguments=0x0000000000000000, p_argcount=0, r_return_value=0x00007fffffffd180, r_call_error=0x00007fffffffd198) const at callable.cpp:69:25
    frame #24: 0x00005555600ef910 godot.linuxbsd.editor.dev.x86_64.llvm.mono`CallQueue::_call_function(this=0x0000555560dd75c0, p_callable=0x00007fffcc04d320, p_args=0x00007fffcc04d338, p_argcount=0, p_show_error=true) at message_queue.cpp:221:13
    frame #25: 0x00005555600f01ba godot.linuxbsd.editor.dev.x86_64.llvm.mono`CallQueue::flush(this=0x0000555560dd75c0) at message_queue.cpp:326:6
    frame #26: 0x000055555d47c2b9 godot.linuxbsd.editor.dev.x86_64.llvm.mono`SceneTree::physics_process(this=0x0000555563578b00, p_time=0.016666666666666666) at scene_tree.cpp:471:33
    frame #27: 0x000055555a864f27 godot.linuxbsd.editor.dev.x86_64.llvm.mono`Main::iteration() at main.cpp:3640:45
    frame #28: 0x000055555a7b5c31 godot.linuxbsd.editor.dev.x86_64.llvm.mono`OS_LinuxBSD::run(this=0x00007fffffffd568) at os_linuxbsd.cpp:958:7
    frame #29: 0x000055555a7a98ec godot.linuxbsd.editor.dev.x86_64.llvm.mono`main(argc=4, argv=0x00007fffffffdb28) at godot_linuxbsd.cpp:74:6
    frame #30: 0x00007ffff7c29d90 libc.so.6`__libc_start_call_main(main=(godot.linuxbsd.editor.dev.x86_64.llvm.mono`main at godot_linuxbsd.cpp:44), argc=4, argv=0x00007fffffffdb28) at libc_start_call_main.h:58:16
    frame #31: 0x00007ffff7c29e40 libc.so.6`__libc_start_main_impl(main=<unavailable>, argc=4, argv=<unavailable>, init=<unavailable>, fini=<unavailable>, rtld_fini=<unavailable>, stack_end=0x00007fffffffdb18) at libc-start.c:392:3
    frame #32: 0x000055555a7a9625 godot.linuxbsd.editor.dev.x86_64.llvm.mono`_start + 37

I don't know what fixed it but it doesn't look related to .NET. To find the change that fixed it you'll have to do a git bisect.

godotengine / godot